Lobbying is an activity that usually does not involve gifting (money, items, etc.) I used to be a lobbyist and we were not allowed to promise to give or to give, to someone we were trying to influence, any matierial or monetary item, in order to persuade them to vote in favor of our viewpoint.
Lobbying involves verbal persuasion which can be backed up by facts, studies, statistics, testimonies, etc.
Bribery is handing you $5,000.00 to vote the way we want. Or something such as passing a timeshare onto you, or financing a trip, etc.
Look at some of the following articles and you will surely find an in-depth answer to your question.
http://cc.bingj.com/cache.aspx?q=lob...99ba07,b8e2bb8
lobby vs bribery - Bing